Something's changed. Where's the Kate Winslet we're used to reading about: the ingenue conducting her interviews collapsed on a sofa unconcerned about appearance, guileless, candid, funny. The Kate Winslet I've just shaken hands with is smooth and shiny in a smart trouser suit, with an entourage of publicists plus make-up artist hovering discreetly in the background. Thank God she's still spilling tobacco from her rollies and opting for still water, claiming fizzy "makes me fart".

Forty minutes later, she's propelled into the bowels of the hotel for another brief exchange. It's a curious situation, but hardly surprising, because we're now dealing with Kate Winslet of Titanic and two Oscar nominations fame (Sense And Sensibility was the other). She's so famous you've probably tripped over cardboard cut-outs of her advertising Titanic videos in your local supermarket.

It's a far cry from our first encounter a year ago in the sweaty, dusty heat of the Moroccan winter sun. She was then filming Hideous Kinky, an adaptation of Esther Freud's autobiographical memoir of a bohemian life, which is released here next month. Winslet had just learned of the death from cancer of her first boyfriend, scriptwriter Stephen Tredre.

On the surface it was business as usual, but in the background frantic producers were trying to fit filming around her return to England for his funeral. This, however, clashed with the LA premiere of Titanic, and the many phone calls and general mayhem implied that she was expected to attend. For an outsider hovering on the sidelines, it was an unpleasant reminder of Hollywood priorities. Winslet opted for the funeral in England.

"Yeah," she says wryly in hindsight, "I suppose I was a bit depressed by that... someone I had spent four and a half years of my life with had just died, and it was just that people would even consider . . . they would be having conversations with me, saying things like 'Look, we understand, this must be a very hard time' and then they'd go on to say, 'but don't you think Stephen would have wanted it?'" She takes a deep breath. "'No he bloody wouldn't. Stephen would have wanted me at his funeral and I'm going to be there.' Yes, I was pretty alarmed by that."

But did the experience make her cynical about the industry? To her credit, apparently not. "You can't be cynical because you have to understand what is being asked of you, and what you're here for. And you draw a line down the middle. You also have to remember life is not about success and status and all that kind of thing, and it never has been for me."

It's a point she's consistently made in interviews, and she says this is due to her parents. "Mum and Dad were very much friends, and up for life. There was no anxiety for anything when I was growing up, they just taught me to be me."

So while the life of her co-star Leonardo DiCaprio has spiralled into a greedily documented excess of fast living ("It's very sad," she says, "the guy should be allowed to do what he wants") and a role deliberately sending up his perceived lifestyle in Woody Allen's Celebrity, Winslet has sidestepped the temptations: "I just go resist, resist, resist, remember who you are, remember where you came from."

Instead her priority has been about gluing back the pieces left shattered by Tredre's death. ("There are very dark moments still") and filming the low budget Hideous Kinky, followed by Jane Campion's Holy Smoke, falling in love, getting married.

She met her husband, Jim Threapleton, on the set of Hideous Kinky, where he was third assistant director. Winslet broaches the subject cautiously: "I don't want to start talking about it, but believe me, before I met Jim I was very much, 'Oh I don't believe in marriage'... but the way I feel about it it's something that had to happen between me and Jim. We don't want to be without each other. Ever." She laughs, remembering her reaction to his proposal. "I wasn't at all surprised, actually, but I did think, 'Wow, I am growing up then.'"

Looking at the polished beauty sitting opposite me, it seems a very long time since she was a struggling actress working in a Hampstead deli. But she's still only 23. She received the phone call that launched her film career in the middle of making a pastrami sandwich, and was so excited she had to go outside for a cry. The film was Peter Jackson's Heavenly Creatures, a dark tale of two schoolgirls whose obsessive friendship leads to murder.

Winslet has always been confident in her choice of roles. Her characters are invariably wilful heroines who defy convention: from the headstrong Marianne Dashwood in Ang Lee's Sense And Sensibility, the resolute Sue Bridehead in Michael Winterbottom's bleak Jude, Ophelia to Kenneth Branagh's Hamlet, to the constrained, caged Rose that helped James Cameron's Titanic become the most successful film of all time. It is easy to believe in her incarnations because Winslet believes in them so totally herself.

And along the way, she's had to grow up in the public eye. This has meant getting used to tabloid titterings about her fluctuating weight, and unwanted interest in boyfriends (snatched pictures of snogs with Rufus Sewell was about as shocking as it got). Some of the reviews for Titanic were depressing, devoting more time to the size of her ankles than to her acting skills. Winslet has developed a thicker skin, which might go some way to explain the honed, reserved personality I'm encountering now, with a barricade of publicists and agents between her and the rest of the prying world.

"This year I've really had to learn to deal with it. You're yourself and then you wish you hadn't been because they go and fuck you up. There was something in Vogue last year which was bloody awful. I was having a really nice time chatting, like we are now, and suddenly it's 'Kate is lying on her sofa eating grapes while everyone is running around her,' and I thought, what a load of absolute shit. But I refuse, absolutely refuse to spend any time worrying about what people have written about me in the papers, because at the end of the day all my family and friends know that it's all crap, most of the time, so why bother. Life is too short."

So for all that Titanic has made her universally recognised, the experience has taken its toll in different ways. The easy candour that shaped earlier interviews has been replaced by a more distant composure.

No more chances then of reading glorious indiscretions, like the occasion she described working with Cameron - "He has a temper like you wouldn't believe... You'd have to pay me a lot of money to work with Jim again" - that landed her in such hot water with Fox at the time. Or other endearing insights; such as her jokey comment when she no longer fitted into a favourite dress - "Then of course, I realised the material had shrunk."

The Kate Winslet of recent years - breathless, gushy with a hint of the Emma Thompson school of jolly hockey sticks - has been banished to the cuttings library. All that energy has been sucked in and channelled in other, more private directions. But that boundless lack of reserve was what made her special. This slick packaging and new-found confidence instead verges on the bland - perfectly friendly and charming, but in truth, fairly disappointing.

What was I expecting? Girly bonding and shared secrets? You're never going to get that in the space of 40 minutes. I don't doubt that it's still there, bubbling under the surface, but dig for any clues and they're brushed aside with easy, noncommittal answers. She likes to walk, she likes to swim, she likes to spend time with her family. There either isn't anything to give away or she's choosing not to. I think she's made a deliberate choice. She doesn't have to sell herself any more; loved up and lovely, Winslet's finally built a wall between her public life and private one.

She's still open in conversation, but she is now more considered, reserved - more adult, perhaps? She's perfectly happy to admit to being clueless about drugs: "I've never touched drugs in my life. Can't bear them. People will talk about them and use 'in' names like Charlie and all that stuff and I will honestly go, 'What's that?' I really don't know what they are. I've never even had a puff of a joint."

She also says she is a hopeless shopper ("I get so irritated, hot and bothered"), claims to have bypassed any potential rebellious streak, and doesn't see herself as a particularly active person. "To be perfectly honest with you I don't know enough about politics. I'd never want to do something just for the show of it." But Winslet quietly involves herself in charities ("Obviously cancer I do a lot for"), and thinks instead about the responsibilities, rather than the temptations, that fame has brought her. "I think I am seen as some kind of a role model now, which is nice, and I'd like for that to do some good."

The Titanic legacy has affected her attitude to work as well. "After that film, everything was so kind of new and odd and exhausting, I thought: Why can't I just act, do my job?" Hence Hideous Kinky, al-together different and on a comparatively tiny scale. Accepting that role was also, surprisingly, about restoring lost confidence. "I think I'd lost the courage to trust my instinct a little bit, because Titanic was so big. There was a lot of pressure on Leo and me all the time to really come up with a profound love story, otherwise it was just going to be a film about a sinking ship."

It had in fact been Tredre who first introduced her to Esther Freud's book, when it was published. "I think he thought I would like this story - with its colour, excitement and challenge, and he was right, I absolutely loved it." So when the script arrived at her flat Winslet was intrigued. She read it and thought, "Bang on the money, perfect," and instigated a meeting with its director, Gillies Mackinnon. "I really felt I needed to have a good laugh. For once it wasn't about wearing a corset, it was running around wearing no shoes . . . which is very me.

"There were so many things about her that were very similar to my upbringing. Not that my parents were hippies, but I remember holidays would be camping and staying with friends in farmhouses, getting in a car and just going. I'll never forget mum waking me up at four in the morning and whispering, 'We're going to Sweden'. It was so exciting."

The end result is a film that's colourful and evocative and Winslet shines as the slightly chaotic young bohemian, tripping her way around Morocco in the sunny seventies with two daughters in tow. The film is an ensemble piece, relying on the two young girls, Bella Riza and Carrie Mullan, as much as Winslet. "They were a handful," she laughs, "but they were georgeous."

Filming Holy Smoke was another happy experience for Winslet. "Jane stood up and did a speech on day one of the shoot. 'I just want to say to everybody it doesn't matter if no one sees this film, let's just pretend it's a rehearsal and have a really great time and see what we come up with,' and I just thought, 'Hurray'." Of course everyone will see the film; it's got Kate Winslet in it . It's about a young Australian woman drawn into a religious cult and de-programmed by Harvey Keitel.

It looks like Winslet can now take time out to settle into married life. She's chosen not to work for a while. Instead it's back to favourite walks, favourite pubs, and visits to her family in Reading. Day-to-day life, she says, has nothing to do with "Kate Winslet, famous actress" life.

"Oh I can still get on with being me," she smiles, brushing yet more tobacco off her lap, "People do have a bit of a stare and a whisper, but they're quite English about it. It's usually when I've just come out of the swimming pool and still have goggle rings around my eyes.

"I still do everything like normal... it just so happens I am very famous now. And still I want to laugh when I say that. It's very daft, how did that happen?"